The present invention relates to pattern inspection and foreign material inspection that detect a defect (including a short circuit and a broken wire) and a foreign material of a pattern to be inspected. More particularly, it relates to an inspected-pattern defect inspection method, and its apparatus, used for inspecting a defect and a foreign material on a pattern to be inspected such as a semiconductor wafer, a liquid crystal display, and a photo mask. The description below is based on the assumption that a defect includes a foreign material.
Japanese Patent Laid-open No. 8-320294 (prior art 1) is known as the prior art relating to defect inspection of a pattern to be inspected. This prior art 1 describes the technology comprising the steps of: in a pattern to be inspected such as a semiconductor wafer where an area with high pattern density such as a memory mat unit and an area with low pattern density such as a peripheral circuit are mixed in a chip, on the basis of the frequency distribution of brightness on the detected image, analog-to-digital converting the detected image signal to obtain a digital image signal, and then gray-scale converting the digital image signal so that brightness or contrast between the high density area and the low density area of the pattern to be inspected becomes a value of the predetermined relationship; making a comparison in a state in which this gray-scale converted image signal is aligned with an gray-scale converted image signal to be compared; and thereby inspecting a minute defect with a high degree of accuracy.
In addition, the technology described in Japanese Patent Laid-open No. 10-78668 (prior art 2) is known as the prior art relating to inspection of a pattern on a photo mask. According to this prior art 2, a UV laser such as an excimer laser is used as a light source. In the description, a mask is evenly irradiated with the UV light, coherence of which is reduced by rotating a diffusing plate inserted into an optical path, and then a feature index is calculated from obtained image data of the mask to judge whether or not the photo mask contains a defect.
Moreover, Japanese Patent Laid-open Nos. 2001-176942, 2001-194323 (U.S. Pat. No. 6,621,571), and 2001-296570 (U.S. application Ser. No. 09/764,457) are further known as the prior art relating to a pattern defect inspection apparatus using UV or DUV laser light. In recent years, in the field of the LSI production, circuit patterns formed on a wafer are microminiaturized as high integration demands. Its pattern width, therefore, is reduced to 200 nm or less, which reaches the resolution limit of an image-formation optical system. Accordingly, an increase in NA of the image-formation optical system is being made, and an optical superresolution technology is being applied.
However, the increase in NA reaches the physical limit. Because of it, making a wavelength used for detection shorter, more specifically, using wavelength bands of ultraviolet light (UV light) and far-ultraviolet light (DUV light), is an essential approach.
On the contrary, as a wavelength of a light source of an exposure apparatus for copying a pattern is made shorter, the shorter wavelength also involves the sensitivity of resist. Accordingly, the sensitivity to an exposure wavelength is the highest. Thus, when an inspection wavelength extremely gets close to the exposure wavelength, the resist reacts at the time of inspection, causing a pattern to shrink or expand. Accordingly, there is a possibility that the pattern will be damaged. It is known that in a typical review using a secondary electron such as SEM, the density of energy applied to a wafer is high, and therefore, a size called shrinkage varies.
Also judging from this, although the energy density is low as compared with the SEM, a wavelength of light to be irradiated is close to, or the same as, an exposure wavelength of the resist. Therefore, it is probable that there is some influence.
Moreover, in a memory part having a narrow line width like a system LSI, or in a chip where a rough wiring part is mixed, irradiating with inspection light causes heat to accumulate in part of the chip. Therefore, there is also concern that the bimetal effect will cause peeling of a pattern called stress migration.
Further, in recent years, if a design rule becomes 100 nm or less, it is necessary to use a material with low conductivity (low-k). In the case of the material with low conductivity, its film becomes softer. This means a decrease in mechanical strength, resulting in low adhesion at the time of deposition. Accordingly, the peeling of a pattern is liable to occur as described above. As the material with low conductivity, many kinds of materials including Cu/SiO2 and Cu/SiOF are used for wiring of Cu. Usually, SiOC and SiON are used to form a barrier layer. Further microminiaturization makes it possible to produce a thinner material with ultra low conductivity, having low ratio conductivity. Because new materials are being developed, thinner materials are used in future. At all events, soft film is a common property. Not only the peeling but also diffusion by heat, chemical change by ultraviolet light of impurities, and the like, may influence the film. Therefore, there is a possibility that a bad influence will be exerted on a device.
In addition, there is a limit of the heat capacity that is acceptable for the device, which is called thermal budget. Taking stress by heat into consideration, this is a period of time during which a device structure does not change. This includes, for example, anneal time, and a step of thermal diffusion. It is also probable that when heat is applied by inspection, the limiting value is exceeded, exerting an influence on a manufacturing process of the device.
Further, also at the time of inspection, it is necessary to perform inspection at high speed. Therefore, it is not possible to use a method in which a thinly narrowed laser beam is scanned on a sample. On the other hand, if illumination with a laser beam is performed with a visual field being fully opened, a speckle occurs. In addition to it, overshoot and undershoot called ringing also occur in the edge part of a circuit pattern. Therefore, a high-quality image cannot be obtained.